It is to be noted that throughout this application various publications are referenced by Arabic numerals within brackets. Full citations for these publications are listed at the end of the specification. The disclosures of these publications are hereby incorporated by reference in their entireties into this application in order to more fully describe the state of the art to which this invention pertains.
Targeted delivery of medically useful elements to a particular site continues to be of considerable importance in diagnosis, prognosis, and therapy of various lesions [1]. For example, gadolinium is used extensively for magnetic resonance imaging (MRI); barium and iodine are used for X-ray computed tomography (CT); technetium, indium, lutetium, samarium, and iodine are used for planar imaging and single photon emission tomography (SPECT); gallium and fluorine are used for positron emission tomography (PET); rhenium, samarium, yttrium, lutetium, and phosphorous are used for radiotherapy; and europium, ruthenium, and rhenium have potential utility for optical imaging and optical tomography. In particular, 32P, a β-emitting radioisotope of phosphorous, has considerable radiotherapeutic potential for various lesions depending on its incorporation into a selected molecular carrier. For example, incorporation of 32P into a steroid receptor binding molecules such as androgens, estrogens, antiestrogens, progestins, and the like may be useful for the treatment of steroid receptor positive tumors; incorporation into somatostatin receptor binding molecules such as octreotide may be useful for the treatment of neuroendocrine tumors; or incorporation into carbohydrate receptor binding molecules such as selectins or integrins may be useful for inflammatory process.
Conventional bioconjugate method of delivering diagnostic and therapeutic agents, both small molecules and macromolecules, such as drugs, enzymes, metal complexes, fluorescent and radioactive probes, and the like to a particular tissue involves the external attachment of these agents to a targeting carrier whose size is typically considerably larger than the effector molecules. This methodology has been referred to as “external bifunctional” approach, and has been quite successful in the development of in vitro diagnostic products. However, one of most vexing problems in bioconjugate chemistry with respect to the development of in vivo diagnostic and therapeutic agents is that the external attachment of a radionuclide complex to small molecule carriers almost always impedes receptor binding [2]. This problem can be readily resolved by the use of macromolecular carriers such as antibodies or reasonably large peptides where the epitope topology is not much altered. An epitope is a specific region of the molecule that is actually involved in the adhesion of the effector and carrier through the intermolecular forces. Unfortunately, macromolecular bioconjugates present many problems with respect to bioavailability, pharmacokinetics, and biodistribution. Moreover, for nuclear medicine applications, the external bifunctional agents presents additional radiotoxicity of critical non-target tissues such as the liver and the kidney due to unacceptably large percent injected dose to and poor clearance from these organs. In order to address both of the aforementioned problems, viz., receptor binding and bioavailability, we have previously introduced a general concept referred to as ‘internal bifunctional’ approach or ‘small molecule drug mimics’ wherein the medically useful atom is integrated into a known effector molecule thereby preserving the overall size and shape of the original molecule. This approach was based on the well established principle that antibodies, enzymes, and receptors are multispecific, i.e., they will bind to molecules that are topologically similar to the natural antigens, substrates, or ligands. The concept of radionuclide metal ion based drug mimics was independently proposed by Rajagopalan [3, 4] and Katzenellenbogen [5]. Katzenellenbogen's work on steroid mimics confirmed experimentally that the idea of integrating a metal ion into natural receptor ligands is a viable strategy for targeted delivery of diagnostically and therapeutically useful radionuclides to target tissues [2, 5–7].
Our previous work on steroid mimics focused on isosteric substitution of metal ions such as technetium and rhenium into various positions in aromatic and non-aromatic steroidal framework as illustrated schematically by generic structures 1 and 2 [4]. A considerable difficulty with respect to incorporating a metal ion into a carbon framework is the deviation
from tetrahedral geometry. For example, Tc(V) and Re(V) oxidation state forms a square pyramidal geometry, which may contribute to reduced receptor binding capability. Furthermore, 186Re obtained from the generator is not carrier free, i.e., only 5% is in the radioactive form; the rest of the material is the non-radioactive (‘cold’) isotope of rhenium. Therefore, this presents a formidable challenge to prepare steroid mimics with very high specific activity needed for radiotherapeutic purposes. In contrast, 32P can be obtained in a carrier-free from. Thus, there is a need in the art to prepare novel radiodiagnostic and radiotherapeutic steroidal compositions with high specific activity and having high affinity for steroid receptors. Accordingly, the present invention focuses phosphorous-based steroid mimics. Although phosphasteroids have been known for sometime [8, 9], surprisingly there has not been much activity in developing them into medically useful products. Introducing a phosphorous atom into the steroidal skeleton solves two key problems: (a) the preservation of tetrahedral geometry, and (b) the preparation of steroid mimics having very high specific activity. In addition, steroids are transported across cell membrane from the blood to the cytoplasm via steroid binding proteins (SBP) [10]. Since the phosphorous containing steroid mimics of the present invention have same topology at the original steroids, it is anticipated that the mimics will bind to SBP and transported into the cell in the same manner as the native steroid molecules.